A flange nut delivers a built-in washer (flange) that distributes the clamping load around a broader region than a daily lock nut. The design won't need a flat washer and is the higher selection for an outsized or slotted hole. Nylon substance in nuts helps prevent loosening due to vibration. https://engineering-variable-freq09742.blazingblog.com/28297073/custom-screw-an-overview